Looks like the cardbus adapter recognizes the USB hub controller on the
  S720, but it doesn't recognize that there's something connected to that
  hub, or the elan card won't allow access.

I don't quite follow your test data, but it seems that

  when plugged in to a regular cardbus slot (not using this elan
  adaptor) the S720 is recognized all the way to the modem/serial part,
  with every machine and both NetBSD and Knoppix on every combo you have
  tried

  when plugged in via the elan adaptor the ohci/uhub appears, but not
  the modem/serial part, and this is true for both NetBSD and Knoppix

At this point I would suspect that there's a need for the ohci to send
an interrupt to indicate that a device has appeared at the hub, and that
this isn't being routed correctly.  But I'm not familiar enough with USB
device attachment to be sure.  It really feels like the elan cardbus
adaptor is somehow odd/defective.

What happens if you plug some other cardbus card, like
ex0/tlp0/ath0/rtw0 into the elan?  Does it work?  If my theory above is
right, it would attach but the interrupts wouldn't work.